NTC thermistors are known to be useful for applications such as temperature compensation or temperature detection. Furthermore, the demand for NTC thermistors with reduced deviations in characteristics has recently been rising as electronic devices have become smaller and circuits have become more complicated. More specifically, for example, the deviation in resistance value was previously acceptable when it was within ±5%, but now is required to be as small as within ±1% to ±0.5%.
Regarding the deviation reduction mentioned above, there are more-detailed requirements: NTC thermistors should be unlikely to change characteristics over time when they are left at high temperatures around 125° C. and, furthermore, they should also be unlikely to change when they are left at further elevated temperatures around 175° C. so that they can be used in automotive applications; and their characteristics should not be significantly affected by unavoidable variations in conditions that may be encountered during the manufacturing process, or in other words, their manufacturing yield should be satisfactorily high.
In particular, the latter affected characteristics that may occur during the manufacturing process as a result of variations in conditions is described in more detail as follows. The characteristics of NTC thermistors are often affected by variations in conditions that occur during the manufacturing process, in particular, variations in the firing temperature used in the firing process. For example, the conditions of the firing furnace, the input (charge quantity) of the unfired chips (i.e., the precursors of the NTC thermistors) into the furnace and their arrangement in the furnace, the weather conditions on the day the firing furnace is operated, and other factors undesirably cause variations among the unfired chips in the firing temperature to which each unfired chip is exposed. This results in the situation where the individual NTC thermistors differ from each other in firing history. The resistance value and other characteristics may thus vary among the resulting NTC thermistors.
As can be seen from this, the characteristics of NTC thermistors generally have relatively high “dependency on firing temperatures.”
On the other hand, there are some measures that can be taken after the firing process to address the variations in characteristics among NTC thermistors. An example is the resistance adjustment method, in which a heat treatment at a temperature of 250 to 500° C. is carried out after the formation of the external electrodes so that the intended resistance value should be obtained. However, the rate of change in resistance value brought about by this heat treatment varies depending on the constitution and shape of the semiconductor ceramic composition used to construct the NTC thermistors. It is thus difficult in some cases to achieve the resistance value that matches the desired value by heat treatment.
By way of an example of the semiconductor ceramic compositions for NTC thermistors interesting for this invention, Japanese Unexamined Patent Application Publication No. 6-263518 (Patent Document 1) discloses a ceramic composition for NTC thermistors represented by the general formula FezNixMn3−x−zO4 (x=0.84 to 1 and 0<z<1.6). Patent Document 1 states that this ceramic composition is characterized by a small rate of change in resistance at elevated temperatures.
However, the ceramic composition described in Patent Document 1 has proved to have high dependency on firing temperatures.
On the other hand, Japanese Unexamined Patent Application Publication No. 2005-150289 (Patent Document 2) discloses a composition for thermistors containing a manganese oxide, a nickel oxide, an iron oxide, and a zirconium oxide, wherein the manganese oxide and the nickel oxide as main ingredients are contained in amounts of a mol % based on Mn (where a is in a range of 45 to 95, excluding 45 and 95) and (100-a) mol % based on Ni, and the iron oxide and the zirconium oxide are contained in amounts of 0 to 55% by weight based on Fe2O3 (excluding 0% by weight and 55% by weight) and 0 to 15% by weight based on ZrO2 (excluding 0% by weight and 15% by weight) relative to 100% by weight of the main ingredients. Patent Document 2 states that this composition can be used at high temperatures and high humidity levels with small rates of change in resistance and, furthermore, that it satisfies various requirements in circuit design because it allows a wide adjustable range of the B parameter on the low-temperature side (25 to −40° C.).
However, the composition for thermistors described in Patent Document 2 has proved to be sensitive to changes in the manufacturing conditions, thereby giving a low yield, and to be not fully reliable especially when it is left at an elevated temperature.
A more detailed explanation can be found in Examples section of Patent Document 2, which discloses a constitution containing main ingredients consisting of Mn: 80.0 mol % and Ni: 20.0 mol % and Fe2O3 in 10.0% by weight relative to 100% by weight of the main ingredients (in other words, a constitution containing Fe in 9.51 parts by mole relative to 100 parts by mole of the main ingredients) as Sample 21 falling within the scope of the invention and also discloses a constitution containing main ingredients consisting of Mn: 80.0 mol % and Ni: 20.0 mol % and Fe2O3 in 30.0% by weight relative to 100% by weight of the main ingredients (in other words, a constitution containing Fe in 28.54 parts by mole relative to 100 parts by mole of the main ingredients) as Sample 22 falling within the scope of the invention.
When placed in an ambient temperature environment of 175° C., however, the constitution of Sample 21 described above undesirably experiences a great change in resistance value; it has proved to be lacking in the reliability in high-temperature environments.
On the other hand, the constitution of Sample 22 described above has proved to be disadvantageous in the following way. With this composition, adjusting the resistance value of the NTC thermistors after the firing process in the way described above, or more specifically by heating them at a temperature of 250 to 500° C., requires a relatively high temperature, often leading to great variations in characteristics after the resistance-adjusting operation, and this makes it difficult to achieve consistent characteristics. In this way, this composition may cause the yield to be reduced.